Owing to advantages of a high energy density, small self-discharge and excellent long-term reliability, lithium ion secondary batteries containing a nonaqueous electrolyte solution are practically used as batteries for compact electronic devices such as a notebook personal computer and a cellular phone. Besides, the lithium ion secondary batteries have been applied to electric vehicles, household storage batteries, and electric power storage.
However, a decomposition product of a solvent contained in the electrolyte solution produced through reductive decomposition on the surface of a negative electrode may deposit on the surface of the negative electrode to increase the resistance, or a gas generated through the decomposition of the solvent may swell the battery. Besides, a decomposition product of the solvent produced through oxidative decomposition on the surface of a positive electrode may deposit on the surface of the positive electrode to increase the resistance, or a gas generated through the decomposition of the solvent may swell the battery. As a result, there arises a problem that battery characteristics are degraded because the storage characteristic of a battery is degraded or the cycle characteristic of a secondary battery is degraded.
In one of methods for solving the above-described problem, a compound having a function to form a protective coating is added to a nonaqueous electrolyte solution. Specifically, decomposition of the compound added to the electrolyte solution is intentionally impelled on the surface of an electrode active material in performing an initial charge operation, so that the resultant decomposition product may form a protective coating having a protective function for preventing the decomposition of a solvent. The protective coating having the protective function for preventing the decomposition of a solvent is called as an SEI (Solid Electrolyte Interface).
Non Patent Literature 1 describes that a chemical reaction or decomposition of a solvent occurring on the surface of an electrode is appropriately suppressed by forming a protective coating on the surface of a negative electrode from an additive, and thus the battery characteristics of a secondary battery are retained. In this literature, however, it is presumed that an SEI is formed on the surface of the negative electrode from the additive, and gas generation and the like through the oxidative decomposition of the solvent on a positive electrode is not sufficiently suppressed.
On the other hand, use of a high-potential positive electrode for realizing a high energy density secondary battery is under examination. Patent Literatures 1 and 2 disclose a lithium ion secondary battery using, as a positive electrode active material, a lithium transition metal composite oxide containing an over-stoichiometric amount of lithium. A high potential lithium ion secondary battery as described in Patent Literatures 1 and 2 has a potential of 4.5 V or more. Therefore, as compared with a lithium ion secondary battery having a general voltage (of 3.5 to 4.2 V), the gas generation through the oxidative decomposition of a solvent is easily caused on a positive electrode. Accordingly, there is a demand for a technique for suppressing the gas generation on a positive electrode in a high potential lithium ion secondary battery.
Patent Literature 3 discloses a method for suppressing the gas generation from a positive electrode by using a silane coupling agent and an epoxy resin for forming a protective coating on the surface of the positive electrode. Besides, Patent Literature 4 discloses a method for suppressing the gas generation from a positive electrode by causing a boric acid compound to adhere to a positive electrode active material. On the other hand, Patent Literatures 5 and 6 disclose a technique to improve the storage characteristic and the like of a battery by adding lithium difluorophosphate to a nonaqueous electrolyte solution.